Glyoxal studied with 'Multimode', explicit large amplitude motion and anharmonicity

We are studying the vibrations of polyatomic molecules using normal coordinates with our code 'Multimode'. Recently we extended 'Multimode' to include one large amplitude coordinate, using the theory of the reaction path hamiltonian together with ab initio calculations to obtain the potential energy surface (and its first and second derivatives). Motion perpendicular to the large amplitude motion was therefore assumed to be harmonic. Now we introduce a quartic forcefield. We treat the effects of this anharmonicity in two ways: (i) by the variational method within 'Multimode', and (ii) by perturbation theory. This latter approach, which may be called 'the vibrational adiabatic approximation', is particularly attractive because of its simplicity. It opens the way for the study of such vibrations using a combination of large amplitude motion theory, variational theory and perturbation theory. We demonstrate the new approach for glyoxal.